In an MR head, the resistive value of an MR element changes relative to the changes of a magnetic field. In other words, an MR element is handled in the circuit construction a variable resistor in which the resistance value changes in response to the strength of the magnetic field that is applied. Because of this, when a constant current flows in the MR element, since the voltage drop that is generated in the said MR element changes relative to changes in the magnetic field, it is possible to detect the magnetic field strength by detecting the voltage drop of the MR element. Also, it is possible to detect the magnetic field strength by applying a constant voltage to an MR element, and detecting the current that flows in said MR element.
The pre-amp that detects the voltage drop of an MR element depends on the application system for the voltage for the MR element. There are two types: a single-end type and a differential type. FIGS. 6 and 7 respectively show the construction of a single-end type and a differential type pre-amp.
As is shown in FIG. 6, the system that uses an amplifier of the single-end type is constructed by means of current supply circuit (10) that supplies current (I.sub.B) to MR element (20), and amplifier (30a).
Current supply circuit (10) supplies current (I.sub.B) that is set in response to a control signal (S.sub.D) to MR element (20), and since the resistance value of MR element (20) changes in response to the magnetic field strength, the voltage of node (ND1) is determined by said resistance value and current (I.sub.B). Since the voltage of node (ND1) is amplified by amplifier (30a), and amplified signal (S.sub.out) is output, the magnetic field strength can be detected corresponding to signal (S.sub.out).
In a system that uses the differential type amplifier shown in FIG. 7, MR element (20) is connected between nodes (ND1) and (ND2). Also, resistive elements (R1) and (R2) are series-connected between nodes (ND1) and (ND2). Current (I.sub.B) is supplied to node (ND1) by current supply circuit (10). On the other hand, current (I.sub.B1) that flows in the direction of node (ND2) to current supply circuit (12) is supplied by current supply circuit (12).
Output current (I.sub.B1) of current supply circuit (12) is controlled corresponding to the voltage of contact point (ND0) for resistive elements (R1) and (R2). Normally, the supply current (I.sub.B1) of current supply circuit (12) is controlled such that the potential of node (ND0) becomes ground potential (GND).
Since the potential difference between nodes (ND1) and (ND2) is amplified by differential amplifier (30), and the amplified signal (S.sub.out) is output, the resistance value for MR element (20) corresponding to said output signal (S.sub.out) can be detected, and, furthermore, the magnetic field strength can be detected.
Because MR element (20) is susceptible to static electricity, it is necessary that the potential of MR element (20) in regard to the circuit construction be considered to make it as close as possible to ground potential (GND). In the system using the differential type amplifier shown in FIG. 7, since the middle point of MR element (20) is maintained at ground potential (GND), electrostatic breakdown is effectively prevented. Also, in the system using the single-end type amplifier shown in FIG. 6, due to the fact that the supply current (I.sub.B) of current supply circuit (10) is set small and the potential of node (ND1) is maintained at a level close to 0.1 to 0.2 V, its electrostatic breakdown can be prevented. Since the single-end type amplifier can be operated with a single power supply voltage, its circuit construction is simple.
Incidentally, with the system that uses the above-mentioned single-end type amplifier, in the event in-phase noise is applied to MR element (20) due to noise, in other words, in the event a noise component of the same phase appears at both ends of MR element (20), since said noise component is amplified by means of amplifier (30a) and appears on output signal (S.sub.out), it is susceptible to in-phase noise. In other words, the common mode removal ratio (CMRR) for this type of amplifier is small.
On the other hand, in principle with the system using the differential type amplifier, the CMRR can be increased up to a high frequency region, but because the middle point of MR element (20) is maintained at almost ground potential (GND), a negative power supply voltage becomes necessary; thus there is the disadvantage that the circuit construction becomes complicated.
This invention was made based on this type of information, and its purpose is to offer an amplifier circuit used for an MR head that is operable with a single power supply voltage, and in which a large CMRR can be maintained.